The deaths of four American horses and illness in six others are being linked by the US Food and Drug Administration (FDA) to drugs compounded by a Kentucky pharmacy to treat the neurological disease Equine Protozoal Myeloencephalitis (EPM).

The FDA said it was investigating two lots of pyrimethamine and toltrazuril drugs compounded by Lexington-based Wickliffe Pharmacy. The batches had been associated with illnesses and deaths in horses in Kentucky and Florida, the FDA said.

One lot was compounded as a paste and one as an oral suspension. All of the products in these lots were accounted for and were no longer in distribution, it said.

“The FDA is working with the pharmacy and state partners to investigate these adverse events,” the agency said in a statement.

“At this time, FDA testing indicates that one lot of product contained higher levels of pyrimethamine than the labeling indicates. Adverse events associated with high doses of pyrimethamine include seizures, fever and death.”

The FDA said it had received reports of adverse events – including seizures, fever and death – involving two horses in Kentucky and eight horses in Florida that were given these products.

Four horses died or were euthanized, while six horses are recovering.

Horse owners, caretakers and veterinarians should report to the FDA any symptoms such as seizure, fever or collapse in horses who might have received high doses of pyrimethamine, the FDA said.

“During the course of this on-going investigation into the adverse reactions, it has come to the attention of FDA that elevated doses of pyrimethamine may have been used by some practitioners with negative results.

“The usual dose of pyrimethamine in horses is 1mg/kg, which has been shown to be safe for the treatment of EPM in an FDA-approved combination product containing pyrimethamine and sulfadiazine.

“These compounded combination products are not approved animal drugs. Additionally, toltrazuril is not approved for use in horses.